In recent years, thin-film photoelectric conversion devices which are formed from gaseous materials by a plasma CVD method have received attention. Examples of such thin-film photoelectric conversion devices include silicon base thin-film photoelectric conversion devices including a silicon base thin-film, thin-film photoelectric conversion devices including CIS (CuInSe2) compounds or CIGS (Cu (In, Ga) Se2) compounds, and the like, and development of these devices are accelerated and their quantity of production is increasingly enlarged. A major feature of these photoelectric conversion devices lies in a fact that these devices have potential that cost reduction and higher performance of the photoelectric conversion device can be simultaneously achieved by layering a semiconductor layer or a metal electrode film on a low-cost substrate having a large area with a formation apparatus such as a plasma CVD apparatus or a sputtering apparatus, and then separating and connecting photoelectric conversion devices prepared on the same substrate by laser patterning or the like.
One structure of such a thin film photoelectric conversion device is a structure of a stacked photoelectric conversion device making effective use of incident light. The structure of the stacked photoelectric conversion device is a structure for splitting an incident light spectrum and receiving the split light spectrum in a plurality of photoelectric conversion layers, and by stacking a plurality of photoelectric conversion layers which use a semiconductor material having a bandgap suitable for absorbing the respective wavelength bands in decreasing order of bandgap from a light entrance side, it is possible to absorb short wavelength light in the photoelectric conversion layer having a large bandgap and long-wavelength light in the photoelectric conversion layer having a small bandgap, respectively. Therefore, sunlight having a wider wavelength band can contribute to the photoelectric conversion compared with a device provided with one photoelectric conversion layer, and therefore it becomes possible to enhance the photoelectric conversion efficiency.
Patent Document 1 discloses a stacked photoelectric conversion device having a first p-i-n junction, a second p-i-n junction, and a third p-i-n junction in this order from the light entrance side, wherein the first p-i-n junction has an i-type layer of amorphous silicon, the second p-i-n junction has an i-type layer of microcrystalline silicon, the third p-i-n junction has an i-type layer of microcrystalline silicon. It is described that by employing such a constitution, it is possible to realize high photoelectric conversion efficiency by effective use of light and reduce impact caused by light degradation of the i-type amorphous silicon, and thus to improve the photoelectric conversion efficiency after light degradation.
As another stacked photoelectric conversion device of a three junction type, a stacked photoelectric conversion device (a-SiC/a-SiGe/a-SiGe), in which amorphous silicon-carbon is used as an i-type layer of first p-i-n junction at the light entrance side, amorphous silicon-germanium is used as an i-type layer of a second p-i-n junction at the light entrance side and amorphous silicon-germanium having a smaller bandgap than that of the i-type layer of the second p-i-n junction is used as an i-type layer of a third p-i-n junction at the light entrance side, is known.    Patent Document 1: Japanese Unexamined Patent Publication No. HEI 11 (1999)243218
In the meantime, in a stacked photoelectric conversion device including a first photoelectric conversion layer, a second photoelectric conversion layer and a third photoelectric conversion layer, stacked in this order from a light entrance side, the lowest photocurrent of the photocurrents generated in the respective photoelectric conversion layers is the output current from the photoelectric conversion device. In order to take out the output current efficiently, it is general to make even the photocurrents of the first photoelectric conversion layer, the second photoelectric conversion layer, and the third photoelectric conversion layer under a condition of light source: xenon lamp, irradiance: 100 mW/cm2, AM: 1.5, and temperature: 25° C. Thereby, irradiation light is used effectively, and a high output is obtained.
However, in the stacked photoelectric conversion device in which the short-circuit photocurrents are made even as described above, a higher output can be obtained during daytime hours, but the output extremely decreases in the morning and evening.
In view of such circumstances, the example embodiment presented herein has been achieved to provide a stacked photoelectric conversion device capable of preventing extreme decrease of the output in the morning and evening.
A stacked photoelectric conversion device of the present embodiment comprises a first photoelectric conversion layer, a second photoelectric conversion layer and a third photoelectric conversion layer stacked in this order from a light entrance side, each photoelectric conversion layer having a p-i-n junction and formed of a silicon based semiconductor, wherein a short-circuit photocurrent of the first photoelectric conversion layer is larger than a short-circuit photocurrent of the second photoelectric conversion layer or a short-circuit photocurrent of the third photoelectric conversion layer under a condition of light source: xenon lamp, irradiance: 100 mW/cm2, AM: 1.5, and temperature: 25° C. (hereinafter, referred to as “standard condition”).
In the following description, the short-circuit photocurrent of the first photoelectric conversion layer, the short-circuit photocurrent of the second photoelectric conversion layer, and the short-circuit photocurrent of the third photoelectric conversion layer will be also referred to as a first short-circuit photocurrent, a second short-circuit photocurrent, and a third short-circuit photocurrent, respectively.
A study was carried out on the cause of the extreme decrease of the output in the morning and evening to find out that the proportion of a short wavelength component in a sunlight spectrum decreases in the morning and evening and therefore the first short-circuit photocurrent gets extremely smaller than the second short-circuit photocurrent and the third short-circuit photocurrent, and the extreme decrease of the output in the morning and evening is attributed to the fact that such smallness of the first short-circuit photocurrent limits the output current from the photoelectric conversion device. On the basis of this finding, it was found that the output current from the photoelectric conversion device is inhibited from being limited by such smallness of the first short-circuit photocurrent by adjusting a thickness or the like of each photoelectric conversion layer so that the first short-circuit photocurrent will be larger than the second short-circuit photocurrent or the third short-circuit photocurrent under the standard condition, and extreme decrease of the output in the morning and evening can be thereby prevented.
Hereinafter, various embodiments will be exemplified.
The i-type semiconductor layers of the first and second photoelectric conversion layers may be amorphous layers, respectively, and the i-type semiconductor layer of the third photoelectric conversion layer may be a microcrystalline layer.
The ratio of the first short-circuit photocurrent to the smaller one of the second short-circuit photocurrent and the third short-circuit photocurrent may be 1.01 to 1.30 times.
The second short-circuit photocurrent may be larger than the third short-circuit photocurrent under the standard condition.
The ratio of the second short-circuit photocurrent to the third short-circuit photocurrent may be 1.01 to 1.30 times.
The thickness of the first photoelectric conversion layer may be 50 nm to 300 nm.
The thickness of the second photoelectric conversion layer may be 100 nm to 1000 nm.
The thickness of the third photoelectric conversion layer may be 500 nm to 20 μm.
The various embodiments shown herein may be appropriately combined with one another.